Field of the Invention
The invention relates to a method of manufacturing a screw thread including a process for partly removing an apex of a ridge in an incomplete thread part formed in a threaded fastener by screw thread cutting. The invention further provides a threaded fastener.
Background Art
In the threaded fasteners, such as bolts, nuts, and setscrews, an incomplete thread part is mostly formed by screw thread cutting. Conventionally, the incomplete thread part is formed with a relief or chamfer part of a screw thread cutting tool during the screw thread cutting in a thread-starting part (or a thread-ending part), that is, a front end part or a rear end part of a complete thread part (one both sides). The incomplete thread part is a part where the shape of the screw thread (height or sectional shape of the screw thread) is incomplete. More specifically describing the incomplete thread part, the height of the screw thread constantly changes from zero height to a regular height of the complete thread part (or from the regular height to zero height) as the threaded fastener is rotated, and the angle and the flank (crest-root connecting surface) of the screw thread are not as stable as the complete thread part.
Comparing the incomplete thread part to the complete thread part, the apex of the ridge is more often formed with sharp points, and the occurrence of burrs at the apex is more likely. It is desirable that the formation of these sharp points and burrs be avoided because they often lead to such an unfavorable event as galling when engaged with another threaded portion.
So far there have been disclosed different techniques for solving these problems of the incomplete thread part. For example, a screw thread cutting tool is moved synchronously with the pitch of a screw thread already formed so that the incomplete thread part is traced and reprocessed (reprocessed to be corrected) (see the Patent Document 1), a sharp apex of the incomplete thread part is cut off at a certain height position so that an even surface is formed in a vertically intermediate section of the incomplete thread part (see the Patent Document 2), or the whole incomplete thread part is removed (see the Patent Document 3).